Under present circumstances, a DVD bonded type optical disk includes, in terms of the constitution of the recording layer, DVD-5 having a recording capacity of about 5 GB with a single recording layer and DVD-9 having a recording capacity of about 9 GB with two recording layers, and DVD-9 having a larger recording capacity is currently being mainstream. The substrate of DVD-9, which is the current mainstream, uses an aluminum compound as a total reflection film, and gold as a translucent reflection film. Since the translucent reflection film must transmit laser, it must be a thin film comparing to the total reflection film, and thus gold, which is a representative example of stable compounds, has been used.
However, since gold is an expensive material, it is being considered that the material of the translucent reflection film is replaced with a silicone compound and a silver compound, which are inexpensive. In order to further increase the recording capacity, investigations for blue laser are being currently advanced. In the case of red laser, there is no problem in transmittance of red laser when the translucent reflection film is gold, silicone, silver or a silver alloy. In the case of blue laser, however, it has a wavelength around 400 nm, and silver exhibits excellent performance in transmittance at that wavelength comparing to the other materials. Therefore, a translucent reflection film using silver is being developed. However, silver is liable to be oxidized comparing to gold, and is chemically unstable. Therefore, a bonded disk using a silver translucent reflection film cannot provide such durability (reliability) that is equivalent to the conventional bonded optical disk using gold as a translucent reflection film. The improvement in durability (reliability) is a significant problem for attaining full-fledged practical application of the silver translucent reflection film.
The invention is to solve the problem with improvement of an adhesive, and an object thereof is to provide an excellent adhesive that can provide such an optical disk that has high durability (reliability) equivalent to the conventional optical disk using a gold translucent reflection film even with an optical disk using a chemically unstable material comparing to gold, such as silver or the like, as a translucent reflection film.